If 5a = 3125, then the value of 5(a - 3) is:

Key idea: express 3125 as a power of 5 to find a.

1. Recognize that 3125 = 5^5, because 5 × 5 × 5 × 5 × 5 = 3125.

2. From 5^a = 3125 = 5^5, conclude that a = 5.

3. Compute 5^(a − 3) = 5^(5 − 3) = 5^2 = 25.

Therefore the value is 25.